The cytolytic mechanism of cloned cytolytic T cells was investigated based on the observation that: (1)\cytolytic T cells assemble membrane lesions/polyperforin 1 and polyperforin 2 and (2)\that the precursor proteins (perforin) seem to be stored in cytoplasmic granules of T cells. Granules were isolated and tested for their biochemical composition, for their cytolytic activityy and for their ability to form polyperforin complexes. In addition, T-cell clones with different specificities were used for granule isolation to address the question as to whether granule proteins are of clonal origin. Isolated T-cell granules are highly cytolytic for all tumor cells tested. Cytolysis by granules is absolutely dependent on CA ions and is inhibited by Zn2+. Granule-mediated lysis is not H2 restricted, and during granule-mediated cytolysis, polyperforins are assembled on target membranes. Granules contain a set of approximately 12 proteins not detectable in other cellular compartments. These cytolytic proteins are identified or very similar in cytololic granules of various T-cell clones; and antisera developed to granules from one clone detect granules from other clones. Immunofluorescence studies show a direct role for granules in the cytolytic process. (IS)\